An image forming apparatus for forming an image by electrophotography includes a charging unit for uniformly charging the photosensitive surface of a photosensitive drum, a latent-image forming unit for forming an electrostatic latent image, which conforms to image information, on the charged photosensitive surface, a developing unit for developing the electrostatic latent image using toner, and a transfer unit for transferring the developed latent image to a printing sheet. The apparatus executes image forming processing successively while rotating the photosensitive drum.
Among the types of photosensitive drums that can be used, an amorphous silicon drum has a higher durability that than of an organic semiconductor drum employed generally in the prior art and has already found practical use in monochrome copiers. However, a characteristic of an amorphous photosensitive drum is that it exhibits sensitivity unevenness ascribable to a variance in the thickness of the photosensitive film, this being a problem related to manufacture. Sensitivity unevenness affects charging and the exposure characteristic and gives rise to irregular density in the image formed. Further, density unevenness in an image is produced not only by sensitivity unevenness of the photosensitive drum but also by a variance in the optical unit inside the image forming unit. The variance in the optical unit referred to here is attributed to the mounting precision of lenses and mirrors within the optical unit. With an optical unit that has a plurality of lasers, the variance manifests itself as a deviation in the images obtained by irradiation with the lasers and as a shift in the optical axis.
Such density unevenness has not posed much of a problem in the past but is now a problem that the market cannot allow owing to the demand for higher image quality in recent years.
In an effort to solve this problem, the conventional approach has been to suppress variance in the optical unit per se, i.e., to suppress variance by raising the mounting precision of the lenses and mirrors in the optical unit, or to select manufactured parts that satisfy certain conditions so that variance is held below a prescribed value, or to improve the finished precision of the component parts. Alternatively, as disclosed in the specification of Japanese Patent Application Laid-Open No. 7-294837, polarized-light correcting means is provided within the optical unit and a correction is applied by the correcting means, thereby uniformalizing the amount of light in scanning so as to suppress density unevenness.
With these examples of the prior art, however, it is necessary to suppress the sensitivity unevenness of the photosensitive drum or to raise the precision of the optical unit and to perform the required adjustments and selections. The result is in increase in the cost of the apparatus.